A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A boost converter is one example of a switched-mode power converter that is typically employed in off-line applications wherein power factor correction, total harmonic distortion (THD) reduction and a stable, regulated voltage are desired at the output of the power converter. The boost converter may also be employed in DC-DC applications, such as portable and stationary digital signal processors and telecommunications equipment.
A non-isolated boost converter generally includes a boost inductor and power switch coupled across an input of the boost converter. The boost converter further includes a rectifying diode coupled to a node between the boost inductor and the power switch. The boost converter still further includes an output capacitor coupled across an output of the boost converter. The output capacitor is usually large to ensure a constant output voltage. A load is then connected in parallel across the output capacitor. The output voltage (measured at the load) of the boost converter is always greater than the input voltage.
The boost converter generally operates as follows. The power switch is closed for a first interval D. The rectifying diode is reverse-biased, isolating the output capacitor and, therefore, the load from the input of the boost converter. During this interval, the input voltage supplies energy to charge the boost inductor and the inductor current increases. Since the load is isolated from the input voltage, a stored charge in the output capacitor powers the load. Then, for a second interval 1-D, the power switch is opened. The inductor current decreases as energy from both the boost inductor and the input flows forward through the rectifying diode to charge the output capacitor and power the load. By varying a duty cycle of the power switch, the output voltage of the boost converter may be controlled.
The boost converter may be operated in three modes: continuous conduction mode (CCM), discontinuous conduction mode (DCM) or critical mode (CM). The modes are defined by characteristics of the inductor current. More specifically, in CCM, the inductor current is unidirectional and is always greater than zero. In DCM, the inductor current is unidirectional and is equal to zero for a period of time during each switching cycle. In CM, the inductor current is unidirectional and reaches zero only for an instant during each switching cycle.
Analogous to other types of power converters, the boost converter is subject to inefficiencies that impairs its overall performance. More specifically, the power switch and rectifying diode may be subject to conduction losses that reduce the efficiency of the boost converter. Additionally, the power switch [e.g., a metal-oxide semiconductor field-effect transistor (MOSFET)] is subject to switching losses that occur when a charge built-up in a parasitic capacitance of the power switch is dissipated during turn-on. Furthermore, if the boost converter is operated in CCM, the rectifying diode is also subject to a reverse recovery condition during a turn-on interval of the power switch that induces a substantial current spike through the power switch and the rectifying diode. The losses associated with the power switch and rectifying diode increase linearly as the switching frequency of the boost converter increases. Therefore, minimizing the losses associated with the boost converter and, more specifically, the reverse recovery and switching losses associated with the rectifying diode and power switch will improve the overall efficiency of the boost converter.
The losses associated with reverse recovery of the rectifying diode may be reduced by introducing an ancillary circuit coupled to the rectifying diode and the power switch. One example of an ancillary circuit includes a series-coupled ancillary inductor and ancillary switch, coupled across the power switch, that provides a discharge path for the inductor current. The ancillary switch is closed a short time before the turn-on of the power switch to divert the rectifying diode reverse recovery current and the inductor current into the ancillary inductor. Since the ancillary circuit is coupled across the power switch, the ancillary circuit also discharges the parasitic capacitance of the power switch, thereby allowing the power switch to be turned on with substantially zero volts thereacross. Once the power switch is closed, the energy stored in the ancillary inductor may be recovered into the output capacitor by other components of the ancillary circuit.
While the ancillary circuit described may reduce losses in the rectifying diode and the power switch, the addition of the ancillary circuit increases both the complexity and cost of the boost converter. Furthermore, the components of the ancillary circuit are subject to the same current and voltage stresses as the main circuit components during at least part of the switching cycle. Moreover, the components of the ancillary circuit are also subject to switching and conduction losses. Although the ancillary circuit allows the power switch to operate with substantially zero voltage switching (ZVS), the ancillary switch is hard switched.
Accordingly, what is needed in the art is a boost converter topology that overcomes the deficiencies of the prior art.